Pathophysiology
Clinical meaning
Osteoporosis results from an imbalance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation, leading to decreased bone mineral density (BMD), microarchitectural deterioration, and increased fracture risk. Normally, the RANK/RANKL/OPG system regulates bone remodeling: RANKL (from osteoblasts) activates RANK receptors on osteoclast precursors promoting differentiation and bone resorption, while osteoprotegerin (OPG) acts as a decoy receptor for RANKL, inhibiting osteoclastogenesis. Estrogen deficiency post-menopause upregulates RANKL and suppresses OPG, accelerating bone loss at 2-3% per year in the first 5-7 years. Bisphosphonates bind hydroxyapatite crystite in bone matrix and are internalized by osteoclasts during resorption, inhibiting the mevalonate pathway enzyme farnesyl pyrophosphate synthase, inducing osteoclast apoptosis. Denosumab is a monoclonal antibody against RANKL that mimics OPG, potently suppressing osteoclast activity. Fibromyalgia involves central sensitization of the pain processing system with augmented pain facilitation and diminished pain inhibition in the CNS, producing widespread pain, fatigue, and cognitive dysfunction.